Effect of molecular weight on electronic, electrochemical and spectroelectrochemical properties of poly(3,3″-dioctyl-2,2′∶5′,2″-terthiophene)

Abstract

Poly(3,3″-dioctyl-2,2′∶5′,2″-terthiophene), obtained from its corresponding monomer by oxidative polymerization with FeCl₃, has been fractionated into five fractions of reduced polydispersity, covering the M_n range from 1.50 kDa to 10.50 kDa (measured vs. polystyrene standards). The effect of M_n on spectroscopic, electrochemical, spectroelectrochemical and electrical transport properties has been investigated. Fractions of growing M_n show an increasing bathochromic shift of the band originating from the π–π* transition in the neutral polymer with the appearance of a clear vibrational structure for the two highest molecular fractions. The onset of oxidative doping determined from the cyclic voltammogram shifts towards lower potentials with increasing molecular weight. A similar trend is observed for doping induced near infrared bands, which shift towards lower energies (higher wavelengths) with increasing molecular weight and appear at lower potentials in spectroelectrochemical experiments. Finally, a comparison of the FET mobility in two transistors fabricated under identical conditions from polymer fractions differing in their molecular weight shows that a ca. fourfold increase of M_n (from 2.40 kDa to 10.50 kDa) results in a two orders of magnitude increase in the carriers’ mobility (from μ_sat = 4 × 10⁻⁵ cm² V⁻¹ s⁻¹ to μ_sat = 2 × 10⁻³ cm² V⁻¹ s⁻¹). The obtained results underline the importance of the control of the macromolecular parameters in the preparation of electronic and electrochemical devices from poly(3,3″-dioctyl-2,2′∶5′,2″-terthiophene).